1. Field of the Invention
The present invention relates to a method for reduction of rebreathing of gas from dead space and a device for practicing the method. Dead space means the volume of used gas, which during the expiration has filled the airways and is brought back to the alveolus at the following inspiration.
2. Description of the Prior Art
Several diseases result in difficulties or makes it impossible for the patient to achieve sufficient ventilation for adequate gas exchange of the alveolar space in the lungs. This results in the blood, which leaves the lungs, not having appropriate content of oxygen and carbon dioxide which often is referred to as respiratory insufficiency. Lack of oxygen can usually be rectified by supplying to the patient breathing gas having a higher oxygen content than that present in air, but for carbon dioxide a positive effect cannot be achieved in a corresponding way. At serious respiratory insufficiency, respiratory treatment is applied to ensure the patient a sufficient alveolar ventilation. However, if the function of the lungs is seriously disturbed respiratory treatment can result in the pressure in the airway during injection of the breathing volume being very high, which can then present a risk that the airway and the lung tissue are further damaged, a phenomenon often called barotrauma. In order to reduce the requirements of ventilation drastic actions are taken such as narcosis and muscle relaxation, reduction of the body temperature and extracorporeal elimination of carbon dioxide, but such measurements are extremely resource demanding and of limited medical utility.
Positive pressures in the airway also result in other disadvantages except barotrauma: at circulation disturbances the circulation is often retarded in a detrimental way by positive intrapulmonal pressures. In such cases it is important to try to reduce the positive pressure and a method for achieving this is to decrease the volumes by which a patient is ventilated in a respirator.
A well-known principle for treatment of respiratory insufficiency is to decrease the dead space, and this can be achieved by the patient being tracheotomied, which means that on the throat a ventilation aperture is made which connects the trachea with the outer space, but it is obvious that this measurement has significant disadvantages.
Another known principle for treatment of respiratory insufficiency consists in a certain amount of breathing gas being supplied during the respiratory treatment through a special gas tube to a point in the airway during the latter part of the expiration, the supplied gas volume displacing used breathing gas from the airway above the aforementioned inlet point and the dead space then being decreased. This principle, called airway flushing, is useful but has certain limitations and even in some cases certain disadvantages. Thus, the extra gas supply gives some increase of the airway pressure, which at severe circulation insufficiency can be injurious. Principally, in the case of obstructive airway disease, an abnormal high flow of gas from the alveolus occurs during the entire expiration. Then, if it is attempted to free the airway from used gas by means of airway flushing, the positive effect is partly lost due to the fact that the continuous flow of gas from the alveolus tends to fill again the airway with used gas. A large dilution of the gas flow from the alveolus can be achieved by flushing with a high flow, but then other disadvantages arise, for example further increased airway pressure. Another limitation of the known methods and devices for airway flushing is that they require a respirator and, therefore, cannot be used by patients who are not connected to a respirator.